1. Field Of The Invention
The present invention relates to a process for preparing an ion exchange membrane which can advantageously be employed to separate the anode and cathode compartments of an electrolytic cell, especially a chlor-alkali electrolytic cell.
More particularly, the present invention is concerned with a process for preparing an ion exchange membrane in which a precursor membrane of a fluorinated resin containing a sulfonyl halide group as a precursor ion exchange group is contacted with an aqueous solution containing specific amounts of at least one alkali metal or alkaline earth metal hydroxide and at least one specific water soluble organic compound to thereby hydrolyze the precursor group while causing a specific volume increase of the membrane. The ion exchange membrane which can be obtained by the process of the present invention is free of the wrinkles formed during the hydrolysis step of a conventional process and during the electrolytic operation, so that the membrane can ensure advantageously low cell voltage and stable, prolonged electrolytic operation.
2. Discussion Of Related Art
The use of an ion exchange membrane of a fluorinated polymer having carboxylate and/or sulfonate ion exchange groups as a membrane for separating the anode and cathode compartments of an electrolytic cell, especially a chlor-alkali electrolytic cell, has been increasing year after year. In this field of application, it is desired that the ion exchange membrane exhibit low electrolytic cell voltage and high current efficiency, thereby enabling the electrolytic cell having the membrane incorporated therein to be stably operated with low electric power supply.
The above-mentioned ion exchange membrane of a fluorinated polymer having carboxylate and/or sulfonate ion exchange groups is conventionally manufactured by first molding into a film, a fluorinated polymer having precursor ion exchange groups and having thermoplastic properties, and then hydrolyzing the precursor ion exchange groups thereof to thereby form carboxylate and/or sulfonate ion exchange groups. The conventional processes for hydrolyzing the precursor ion exchange groups include a process comprising contacting the precursor groups with an aqueous solution of an alkali metal hydroxide. In this process, an aqueous solution having a relatively high temperature is preferably employed because it contributes toward an increase in the hydrolysis rate of the precursor ion exchange groups. With respect to this process, reference is made to, for example, Japanese Patent Application Laid-Open Specification No. 61-19638. According to the process of this reference, hydrolysis is conducted at a temperature of from 70.degree. to 90.degree. C. for 16 hours, using an aqueous solution containing sodium hydroxide in an amount of from 20 to 25% by weight, based on the weight of the solution.
The conventional processes for hydrolyzing the precursor ion exchange groups also include a process comprising contacting the precursor groups with a mixture of an aqueous solution of an alkali metal hydroxide and a water soluble organic solvent, such as dimethyl sulfoxide and an alcohol, e.g., methanol, ethanol or propanol. Such a mixture is used to swell the ion exchange membrane and accordingly increase the rate of hydrolysis of the precursor ion exchange groups. With respect to this process, reference is made to, for example, Japanese Patent Application Laid-Open Specification No. 57-139127. According to the process of this reference, hydrolysis is conducted at a temperature of 90.degree. C. for 1 hour, using an aqueous solution containing from 11 to 13% by weight of potassium hydroxide and 30% by weight of dimethyl sulfoxide.
In the above-mentioned conventional process for hydrolyzing the precursor ion exchange groups in which use is made of an aqueous solution of an alkali metal hydroxide not containing a water soluble organic compound, the ion exchange membrane substantially does not swell at the hydrolysis step, and in this process, an extremely prolonged period of time is disadvantageously taken to complete the hydrolysis. In addition, it is noted that when electrolysis is conducted for a prolonged period of time using this ion exchange membrane, the ion exchange membrane is likely to swell, thereby forming disadvantageous wrinkles on the surface of the membrane. On the other hand, in the above-mentioned conventional process for hydrolyzing the precursor ion exchange groups in which use is made of a mixture of an aqueous solution of an alkali metal hydroxide and a water soluble organic solvent, it is likely that the ion exchange membrane is swelled to exhibit a marked volume increase, for example about 60% or more, during the hydrolysis, thereby forming wrinkles on the surface of the membrane. The wrinkle formation is likely to be accompanied by problems, such as cell voltage increase attributed to the retention of evolved gas and/or electrolyte by the wrinkles, and such as pinhole formation and membrane tearing attributed to the rubbing of the wrinkled membrane against an electrode. Further, when the wrinkles are formed during the hydrolysis, it is extremely difficult to remove them by post-treatment. As apparent from the above, all of these conventional processes have inherent drawbacks which limit their application.